1. Field of the Invention
The present invention relates to a power source device capable of correctly indicating the remaining capacity of a battery and to an electronic device using the power source device.
2. Description of the Related Art
In recent years, portable electronic devices such as notebook-type personal computers, portable telephones and the like have been extensively used. Such electronic devices use a battery as a power source. In general, however, a secondary, or rechargeable battery such as a NiCd (nickel-cadmium) battery, NiMH (nickel-metal hydrogen) battery, lithium ion (Li+) and the like is used from the viewpoint of operation cost of the device, current capacity that can be obtained instantaneously, etc. In many cases, furthermore, the electronic device contains a charging circuit so that the secondary battery contained therein can be easily electrically charged by simply connecting an AC adapter to the electronic device.
When used on a desk, the portable electronic device operates from electric power supplied from an external DC power source such as an AC adapter or the like, and the user may use the device without concern about the remaining capacity of the battery. When the user carries the device with him, however, the device operates on power from the battery only. Therefore, the user must monitor the remaining capacity of the battery. In a data processor such as a notebook-type personal computer, in particular, the data being processed may be deleted if the battery is depleted. Therefore, the user must remain aware of the remaining battery capacity and be sure to save data on a nonvolatile memory medium such as a floppy disk before the battery is depleted.
To avoid this trouble, some recent notebook-type personal computers are equipped with a remaining capacity display function for letting the user know the remaining capacity of the battery. The method of estimating the remaining capacity of the battery can be divided into two types; i.e., a method of estimating the remaining capacity by subtracting the electric power that is used from the capacity of the battery and a method of estimating the remaining capacity by relying upon the voltage of the battery.
In the case of a battery (lithium ion secondary battery (Li+) and the like battery) having a characteristic that the battery voltage drops in proportion to the amount of electric discharge, it is possible to estimate the remaining capacity of the battery by providing a circuit that measures the battery voltage.
A power source device equipped with such a battery voltage-measuring circuit is constituted as described below. The power source device has a connector which receives a DC electric power from an external unit such as an AC adapter. The battery is held in a battery unit and is removably mounted in the power source device. The external DC power source and the battery are connected to the input side of a DC-DC converter which converts the external DC power or the battery voltage into a voltage needed by the load of the electronic device. The output of the DC-DC converter is connected to the load in the electronic device. Provision is made of a charging circuit for electrically charging the secondary battery from the external DC power source. A blocking diode is connected between the connector and the DC-DC converter in order to prevent the electric power of the battery from leaking to the external side. When the external DC power is used, furthermore, a blocking diode is connected between the battery and the DC-DC converter in order to prevent the electric power from being fed to the battery from the external DC power source.
The voltage across the terminals of the battery is divided by voltage-dividing resistors and is input to a voltage-measuring circuit. The remaining capacity of the battery is indicated based upon the battery voltage measured by the voltage-measuring circuit. When the battery is connected to the power source device, the voltage-measuring circuit correctly measures the battery voltage. When the external DC power source is used and the battery is removed from the power source device, however, the voltage of the external DC power source appears on the voltage-measuring circuit. The blocking diode between the external DC power source and the battery as described above, prevents current from flowing into the battery from the external DC power source. In practice, however, the blocking diode permits a reverse leakage current to flow, and a voltage appears on the voltage-measuring circuit. The voltage-measuring circuit measures the voltage as a battery voltage. Therefore, there occurs an erroneous operation in that a remaining amount of power of the battery is indicated despite the fact that the battery has not really been connected to the power source device. To prevent the occurrence of erroneous operation, a means can be employed in which the resistance of the voltage-dividing resistors for measuring the voltage is decreased with respect to the reverse impedance of the blocking diode. When the voltage-dividing resistors have a decreased resistance, however, the electric power is consumed in an increased amount when the cell is being used, resulting in a decrease in the time for using the battery.